Laser beam emission control for electrophotographic device

ABSTRACT

In an electrophotographic device, a method is provided in which laser power is regulated at predetermined two points of gradation. More specifically, a first laser power regulating means which regulates the magnitude of laser power output and a second laser power regulating means which regulates the slope of laser power gradation characteristic are introduced. Thereby, stabilization of the laser power output value is realized at any stages of the gradation and an electrophotographic device can be provided which reduces variability in image quality between electrophotographic devices.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an electrophotographic device, and in particular, relates to a laser power regulating means for laser control and a method therefor.

[0003] 2. Conventional Art

[0004] With regard to laser power setting for an electrophotographic device a control of the laser beam emission amount is performed by making use of a monitor current Im which is generated at a monitor use pin photo diode provided in a laser diode. More specifically, after converting the monitor current into a voltage the output thereof is controlled by feeding back the same to a regulation circuit in a laser power output control means. In the regulation circuit, the current to be flown into the laser diode itself is controlled so as to keep the fed back voltage at a predetermined value. In order to convert the monitor current to a voltage a resistor is added, however, because of variability in monitor current characteristic of a laser diode, it is generally performed a fine adjustment by making use of a variable resistor.

[0005] Further, in order to control the laser power variably, a reference signal is fetched and the gradation of the laser power is set. Depending on the condition and environment of the electrophotographic device the laser power is varied to thereby stabilize the image quality thereof.

[0006] In the laser beam emission control means provided with the above conventional gradation the regulation center of the laser power is positioned at the center of the gradation, and the regulation is performed by the added variable resistor for converting the monitor current to a voltage so as to obtain a predetermined output value. With regard to the gradation characteristic of laser power regulated with such method, the laser power variability between electrophotographic devices increases as the regulation moves away from the center gradation. Because there is a variability in the characteristic between monitor current and slope efficiency (variation rate of laser beam emission amount with respect to the current flowing through a laser diode) of the laser diode, and the slope of the laser power gradation is determined depending on the above characteristic. As a result, there arises a problem that such as tint of final output images between electrophotographic devices differs.

SUMMARY OF THE INVENTION

[0007] An object of the present invention is to stabilize laser power output value at any stages of the gradation and to reduce variability in image quality between electrophotographic devices.

[0008] In order to resolve the above conventional problems, the present invention proposes a method in which laser power is regulated at predetermined two points of gradation. More specifically, in the present invention, a first laser power regulating means which regulates the magnitude of laser power output and a second laser power regulating means which regulates the slope of laser power gradation characteristic.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic diagram showing a circuit structure of a laser beam emission control means;

[0010]FIG. 2 is a laser power gradation setting circuit of the present invention;

[0011]FIG. 3 is a diagram for explaining voltage setting in the laser power gradation setting circuit;

[0012]FIG. 4 is a schematic diagram showing a structure of an optical unit including a laser beam emission control means;

[0013]FIG. 5 is a schematic diagram showing an entire structure of an electrophotographic device;

[0014]FIG. 6 is a laser power gradation characteristic when regulation is performed according to the method of the present invention;

[0015]FIG. 7 is a laser power gradation characteristic when regulation is performed according to the conventional method;

[0016]FIG. 8 is a characteristic of laser diode drive current versus laser power; and

[0017]FIG. 9 is a characteristic of laser diode monitor current versus laser power.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0018] An embodiment of the present invention will be explained with reference to FIGS. 1 through 9.

[0019] At first, an example of an entire structure of an electrophotographic device to which the present invention is applied is shown in FIG. 5. A photosensitive belt 2 on the surface of which an organic photosensitive conductive material (OPC) is coated is rotatably driven in the arrowed direction during image forming operation. A belt cleaner 3 removes remaining toner on the photosensitive belt 2 after completing a toner image formation. A charger 4 charges electric charges necessary for forming an electrostatic latent image on the surface of the photosensitive belt 2. Laser beams emitted from an optical unit 1 expose the charged surface of the photosensitive belt 2 to form an electrostatic latent image thereon. Developers 5˜8 develop the electrostatic latent image formed on the photosensitive belt 2 by respective color toners successively in the following order firstly by black toner, secondly by cyan, thirdly by magenta and fourthly by yellow. An intermediate transfer drum 9 rotates through contact with the photosensitive belt 2, and the toner image formed on the surface of the photosensitive belt 2 is transferred on the intermediate transfer drum 9 in the order of black, cyan, magenta and yellow (first transfer) to overlap the same and to form a color toner image. A transfer roller 10 transfers the color toner image formed on the intermediate transfer drum 9 on to a recording medium 14 by applying electric field of opposite polarity from the back side of the recording medium 14 (second transfer). A fixing unit 12 melts and fixes the toner transferred on the recording medium 14 by heating and pressing. A drum cleaner 13 removes toner remaining on the intermediate transfer drum 9 after the color toner image on the intermediate transfer drum 9 has been transferred on to the recording medium 14.

[0020] Now, the structure of the optical unit 1 will be explained with reference to FIG. 4. Actually a laser diode 15 is mounted on a circuit substrate in a laser beam emission control means 16, and such as ON/OFF of laser beam emission and laser power regulation thereof are performed by a command from a main control means 17. The emitted laser beams make incident into a collimator 19 to be converged therein. The converged laser beams pass a slit 20 and are irradiated on the mirror face of a polygon scanner motor 18. A drive signal for the polygon scanner motor 18 is sent from the main control means 17 via the laser beam emission control means 16. The laser beams scanned through rotation of the polygon mirror pass through an F-θ lens 21 and a cylinder lens 22 and are irradiated on the photosensitive belt not shown. The head of the laser beams for every one scanning makes incident to a BDT detection means 24 after being reflected by a reflection mirror 23. The BDT detection means 24 functions to create a home position signal for starting image data transmission of one line. The home position signal is also input to the main control means 17 via the laser beam emission control means 16.

[0021] Detail of the laser beam emission control means 16 will be explained with reference to FIG. 1. An operation of laser beam emission control itself is performed by a dedicated laser control IC (laser power output control means) 27. The side of a laser element 25 of the laser diode 15 is connected to an LD terminal and the side of a pin photo diode 26 thereof is connected to a PD terminal. A variable resistor A (a first laser power regulation means) 29 is connected between terminals Vm and VmG. A laser power gradation setting means 28 for setting the laser power gradation is connected to a terminal Vr. The laser control IC 27 is controlled by a state command signal through terminal S/H (regulation/hold). Laser beam emission is performed by ON/OFF of VIDEO signal. An operation of setting the laser power output at a predetermined value is performed by bringing the laser control IC into a regulation mode at first by shifting the state command S/H into L. Subsequently, through shifting the VIDEO signal into L laser beams are emitted. In that, current is drawn into the LD terminal to cause the laser element 25 laser beam emission. When the laser beams are emitted, a monitor current is caused to flow through the pin photo diode 26 depending on the laser beam emission amount. The monitor current flows into the variable resistor A 29 via the laser control IC 27 to generate a voltage thereat. The laser control IC 27 automatically controls the current value drawn in from the terminal LD so that the gradation data voltage inputted at the terminal Vr assumes a same voltage value as the monitor voltage Vm. When the voltages Vr and Vm assume a same value, the laser diode emits beams at a constant power, thus, the state command signal S/H is rendered to H to bring the laser control IC in a hold state. From this moment on, when repeating ON/OFF of the VIDEO signal the laser diode 15 can emit beams at a constant power. The laser power can be set either by a method in which the monitor current is varied by the variable resistor A 29 or by a method in which the voltage value Vr representing the gradation data is varied.

[0022] A detailed circuit diagram of the laser power gradation setting means 28 will be explained with reference to FIG. 2. The present circuit is a current voltage conversion circuit making use of an OP amplifier 31. Resistors 32 and 33 set a reference voltage Va. Resistors 38˜41 have roles of weighting for converting digital signals of REF1˜4 into analogue signals. Transistors 34˜37 are turned ON/OFF by the REF1˜4 to set 16 gradations of 4 bits. Number of REF signals is not limited to four, and if number of bits is increased, the stages of the gradations can be increased. The variable resistor A 30 is the second laser power regulating means of the present invention. In the conventional method, since an invariable resistor was used for the corresponding resistor, the laser power gradation depends on the characteristic of a laser diode itself. A laser diode characteristic which greatly affects the gradation characteristic is a monitor current characteristic as shown in FIG. 9. With regard to 5 mW class laser diodes of respective manufacturers, specifications of monitor current for obtaining laser power 3 mW are in arrange of 0.3˜0.9 mmA. This range looks narrow at first glance, however, the range is very broad for the laser power regulation, and variability of laser diodes for every lot is significant. Further, as will be seen from a laser drive current characteristic as shown in FIG. 8, the laser power variation rate with respect to laser drive current (slope efficiency) affects not a little on the gradation setting.

[0023]FIG. 7 shows laser power with respect to gradation level, when the regulation is performed according to the conventional method. In the conventional regulation method, the regulation is performed only by making use of the variable resistor A 29 (in the conventional method the resistor 30 is an invariable resistor) based on the REF data at a notch 8 corresponding to the center of the gradation, thereby, variabilities as shown by a dotted line and a broken line with respect to an ideal solid line are caused.

[0024] In order to resolve the above problem, the conventional invariable resistor 30 is replaced by the variable resistor B so that the slope of the voltage inputted as the gradation data can be varied to match the laser diode characteristic. FIG. 3 shows a manner of variation of the gradation data voltage. The regulation is performed at two points in that at the maximum and minimum points of the laser power gradation as shown in FIG. 6. Because of the circuit structure, at first an influence of REF data is nulled, then, regulation of the maximum point of Vr voltage, namely the point A of the maximum gradation value is performed by the variable resistor A 29 serving as the first laser power regulation means. Subsequently, the regulation of the point B of the minimum gradation value which corresponds to a condition in which all of REF data are inputted is performed by the variable resistor B 30 serving as the second laser power regulation means. Accordingly, through regulating the two parameters of magnitude and slope of the laser power, a gradation characteristic which is free from characteristic variability inherent to laser diodes can be obtained.

[0025] According to the present invention, since the laser power is regulated at between two predetermined gradation points as in the above embodiment, the gradation variation characteristic is freed from characteristic variability inherent to the laser diode, thereby, a possible variability in laser power gradation characteristic between electrophotographic devices can be reduced. As a result, the present invention is advantageous for stabilizing image quality which is a final output. In particular, the present invention is very effective for a system in which an image quality is stabilized while varying the laser power depending on such as the operating condition and the use environment of the electrophotographic device.

[0026] Further, any laser diodes of a standard specification of a variety manufacturers can be used, and no special specifications are required for the laser diodes which is advantageous for enhancing mass productivity and for cost reduction. 

1. An electrophotographic device comprising: a laser beam emission control means which turns ON/OFF a laser diode by image data signals; an optical unit which converges the laser beam into a predetermined spot diameter to perform scanning; a photosensitive body; a developer which develops an electrostatic latent image formed on the photosensitive body with toner; an intermediate transfer body for overlapping the toner images formed on the photosensitive body; a transfer means which transfers the toner images onto a recording medium; and a fixing unit which fixes the toner images on the recording medium by heating and pressing, wherein the laser beam emission control means is provided with a laser power output control means which controls the laser power at a predetermined output and a laser power gradation setting means which sets gradation of the laser power, and, in order to reduce variability in laser power gradation characteristic between electrophotographic devices, further provided with a first laser power regulating means which regulates magnitude of the laser power output and a second laser power regulating means which regulates slope of the laser power gradation characteristic, and wherein the laser power regulation is performed at between predetermined two gradation points. 